Abstract
Trichosporon jirovecii yeast cells are used for the first time as a source of
l-cysteine desulfhydrase enzyme (EC 4.4.1.1) and incorporated in a biosensor for determining
l-cysteine. The cells are grown under cadmium stress conditions to increase the expression level of the enzyme. The intact cells are immobilized on the membrane of a solid-state Ag
2S electrode to provide a simple
l-cysteine responsive biosensor. Upon immersion of the sensor in
l-cysteine containing solutions,
l-cysteine undergoes enzymatic hydrolysis into pyruvate, ammonia and sulfide ion. The rate of sulfide ion formation is potentiometrically measured as a function of
l-cysteine concentration. Under optimized conditions (phosphate buffer pH 7, temperature 37
±
1
°C and actual weight of immobilized yeast cells 100
mg), a linear relationship between
l-cysteine concentration and the initial rate of sulfide liberation (d
E/d
t) is obtained. The sensor response covers the concentration range of 0.2–150
mg
L
−1 (1.7–1250
μmol
L
−1)
l-cysteine. Validation of the assay method according to the quality control/quality assurance standards (precision, accuracy, between-day variability, within-day reproducibility, range of measurements and lower limit of detection) reveals remarkable performance characteristics of the proposed biosensor. The sensor is satisfactorily utilized for determination of
l-cysteine in some pharmaceutical formulations. The lower limit of detection is ∼1
μmol
L
−1 and the accuracy and precision of the method are 97.5% and ±1.1%, respectively. Structurally similar sulfur containing compounds such as glutathione, cystine, methionine, and
d-cysteine do no interfere.